In recent years, personal computers have increasingly utilized coordinate input devices. A coordinate input device is typically configured to include a pen-shaped position indicator and a position detector. The position detector has an input plane via which a user can operate the position indicator to input characters or drawings by moving the position indicator along the input plane or pointing to specific locations on the coordinate input device. The position indicator includes a detecting unit for detecting pen-pressure against the input plane of the position detector when the user operates the position indicator on the coordinate input device.
An example of a conventional position indicator is disclosed in Japanese Unexamined Patent Application No. H04-96212. This document describes a position indicator having a variable capacitor with a capacitance value which varies with respect to the pressure applied to the position indicator. More specifically, the above-identified document discloses a variable capacitor including a dielectric, a first electrode attached on one surface of the dielectric, a second flexible electrode placed on the other surface of the dielectric, a spacing unit providing a space between the second electrode and the other surface of the dielectric, and a pressure or displacement applying unit applying a relative pressure or displacement between the second electrode and the dielectric. According to this document a position indicator includes a tuning circuit having the variable capacitor as part of a circuit element. The position indicator having the configuration disclosed in the publication continuously changes a tuning frequency of the tuning circuit according to the capacitance of the variable capacitor and transmits information on ON or OFF status of a switch to a tablet with subsequent operational information.
Another example of a position indicator is disclosed in Japanese Unexamined Patent Application No. H08-227336. The position indicator described in this document is a stylus pen capable of detecting pen-pressure, that is, a stylus pen having a pressure sensitive mechanism. The stylus pen includes a pen casing with an approximately cylindrical shape, a resonant circuit, and a pressure sensitive mechanism sensing the pen-pressure using a change in inductance generated by causing a relative displacement between a first magnetic body and a second magnetic body having a coil wound thereabout. The stylus pen further includes a pressing member having a contact unit receiving the pen-pressure and a rear end plane located approximately opposite to the contact unit. The rear end plane of the pressing member forms the first magnetic body. A second magnetic body has the coil wound thereabout such that a front end plane thereof is located a predetermined distance from the rear end plane of the pressing member. An elastic material member is arranged between the rear end plane of the pressing member and the front end plane of the second magnetic body. A first supporting unit is provided so as to hold the position of the second magnetic body against the pen-pressure applied via the pressing member and the elastic material member. A second supporting unit supports the pressing member in a non-parallel state when the rear end plane of the first magnetic body and the front end plane of the second magnetic body are not in parallel. The pen-pressure is detected by causing the first magnetic body to approach the second magnetic body to obtain a change in inductance of the coil wound around the second magnetic body. Subsequently, when the stylus pen is inclined against a position detecting plane, the pen-pressure is applied to the pressing member having the first magnetic body, the pressing member is inclined by the second supporting member, thereby causing one side of the first magnetic body to approach the second magnetic body. In this case, the pressing member is pushed so that at least the other surface of the first magnetic body is not separated from the second magnetic body. However, if the pen-pressure is applied to the pressing member in the lateral direction, the pressing member is not sufficiently pushed, and thus the pen-pressure cannot be detected accurately.
In general, writing tools such as pencils or brushes are used for calligraphy by inclining the writing tools and writing with the side surface of lead or brush. Specifically, the user writes with a pen or a brush in this manner to control the pressure applied thereto to express a subtle change in the thickness or density distribution of written lines. In recent years, there has been an increasing demand for a coordinate input device to simulate the aforementioned writing manner.
A position indicator disclosed in the Japanese Unexamined Patent Application Publication No. H04-96212 includes a pressing unit that presses a variable capacitor, which only moves in an axial direction thereof. Consequently, when the user operates the position indicator in an inclined manner with respect to an input plane of a tablet, pen-pressure may not be detected by the tablet. When the position indicator is allowed to contact the input plane of the tablet at right angles, the pen-pressure is applied in the axial direction of the pressing unit of the position indicator, and hence the pen-pressure can be accurately detected based on the displacement of the pressing unit. However, when the position indicator is inclined against the input plane of the tablet, and the pen-pressure is applied in the lateral direction of the pressing unit of the position indicator, the pressing unit of the position indicator is not moved in the axial direction. As a result, pen-pressure may not be detected.